1. Field of the Invention
The present invention relates to a switcher for a signal transmission system of the type used for transmitting signals which are time-division or frequency-division multiplexed on a high frequency carrier, e.g., carrier-frequency terminal equipment for transmitting electric signals.
2. Description of the Background Art
With a communication system including the above-described type of transmitter using a high frequency carrier, it is a common practice to assign a single standby transmitter to several active transmitters for insuring reliable communication lines. When any one of the active transmitters becomes faulty, the transmission line is switched from the faulty active transmitter to the standby transmitter. To switch the transmission line, a switch is interposed between the input terminal of each of the active and standby transmitters and the line terminal on the input side, and between the output terminal and the line terminal on the output side. When one of the active transmitters fails, a drive signal is fed to the switches either manually or automatically.
As the frequency of a high frequency carrier to be transmitted over the transmission line which should be switched as stated above exceeds a certain value, strict impedance matching is required between the switches or the switcher and the individual transmitters. For example, assume a switcher handling digital signals produced by time-division multiplexing thirty-two voice channels and having a bit rate of 2048 bits per second. Then, a prerequisite is that the impedance of the switcher as viewed from the input of the transmitter, i.e., from the input of the switcher, be equal to the characteristic impedance of the input line. If the impedance of the switcher changes every time the switcher is driven, the output signal of the transmitter will fluctuate, preventing the communication line from being reliable.
On the other hand, a switcher heretofore incorporated in carrier-frequency terminal equipment of the type used for handling the above-stated high frequency signals is so constructed as to set up, on the replacement of the faulty active transmitter with the standby transmitter, a circuit for connecting a test signal generator to the faulty transmitter. For this purpose, the individual switches of the switcher are connected in a chain configuration. The chain connects the input line terminal and output line terminal of the faulty transmitter and the input and output terminals of the standby transmitter and connects the test signal generator to the faulty transmitter. This prevents the impedance matched condition from being maintained since the chain of the switches eventually connects, after the replacement of the faulty transmitter with the standby transmitter, a line piece which renders the other end of the chain open to the input end of the standby transmitter. Although the switches may be connected in a matrix configuration in place of the chain configuration, the matrix configuration makes for large arrangement of cables on a rack at both of the input and output line sides, thereby interfering with the miniaturization of the apparatus at the terminal station.